WASHINGTON — It’s been clear, at least since the 2012 election, that the Republican Party has abrogated its role — really, abandoned any interest — in shaping or seriously discussing American foreign policy. But only recently has this indifference shifted into toxic territory, and on Tuesday the fumes formed a poisonous cloud, the likes of which hadn’t been witnessed in decades.

The occasion was the Senate Armed Services Committee’s vote on Chuck Hagel’s nomination as secretary of defense — a nomination now being held up by an unprecedented Republican filibuster.

In the end, Hagel pulled through the committee hearing, but only on a party-line vote (all Democrats in favor, all Republicans opposed) and after a debate that raised doubts less about Hagel than about the modern GOP’s inclination — and the Senate’s ability — to oversee anything as consequential as national security.

Hagel’s Jan. 31 confirmation hearings had been appalling enough — not just for his own lackluster performance, but more for his inquisitors’ bizarrely narrow focus.

A TASTE FOR THE TRIVAL

They asked almost nothing about the issues that will face the next defense secretary: the budget, the roles and missions of the Army, the balance of drones vs. manned aircraft, the size of the Navy, the future of Afghanistan, or the “pivot” from Europe to Asia.

Instead, they hectored the nominee about the adequacy of his fealty toward Israel, his animosity toward Iran, and whether he was right or wrong about the 2007 troop-surge in Iraq.

There was all that in the follow-up session on Tuesday, plus a whiff of paranoia and sedition that’s rarely been cracked open since the days of Joseph McCarthy.

The stench started wafting through the air with the comments of Sen. David Vitter, R-La., who trumpeted the warnings that in 2008 Hagel gave a speech to the Arab-American Anti-Discrimination Committee.

Vitter called for halting the hearings until a video of the speech could be found, to see whether the nominee had voiced extremist or anti-Israeli comments.

Then came Sen. Ted Cruz, freshman Republican from Texas, who seemed to be explicitly angling for McCarthy’s inheritance.

Cruz shuddered that Hagel had made $200,000 over a two-year period from Corsair Capital, which has contracts abroad, yet he could not tell the committee whether any of that money came from a foreign government.

It would be “relevant to know,” Cruz intoned, “if that $200,000 ... came directly from Saudi Arabia, came directly from North Korea. I have no evidence to suggest that it is or isn’t,” but there should be an investigation.

NOT JUST FOR ROOKIES

At that point, Sen. Bill Nelson, D-Fla., lambasted Cruz for having “impugned the patriotism” of Hagel, for accusing him of getting “cozy” with terrorists.

Now Cruz is but a freshman; his idiocies can’t be ascribed to his party as a whole. But Sen. James Inhofe, Okla., is the Armed Services Committee’s top-ranking Republican, and he not only sided with Cruz but snapped back at Nelson’s admonitions.

Hagel’s nomination had been “endorsed” by the Iranian government, Inhofe said. “You can’t get any cozier than that.”

That was too much for Sen. Carl Levin, D-Mich., the usually amiable and tolerant committee chairman. “I have been endorsed by people I disagree with totally,” he said. “I don’t want people who hate me to ruin my career by endorsing me.”

Sen. Claire McCaskell, D-Mo., went further, warning Inhofe and Cruz, in a “have you no shame, senator” moment, to “be careful” with their tactics of character-smear and guilt-by-association.

Even Sen. John McCain, Ariz., the erstwhile Republican leader, seemed abashed by the storm he’d helped unleashed against the nominee a month before.

“I just want to make it clear,” McCain said, “Sen. Hagel is an honorable man. He has served his country. And no one on this committee at any time should impugn his character or his integrity.”

It was reminiscent of the time, on the 2008 campaign trail, when a woman, fired up by the gunplay rhetoric of his running mate Sarah Palin, started going on about the socialist Muslim Barack Obama — and McCain felt compelled to dial down the passion, defending his opponent as a good American.

One wonders, does McCain lie awake at night, gnashing his teeth at the hash that he’s made of his own reputation and the noisome role he’s played in turning his country’s politics into a cesspool?

Still, McCain’s move to reticence had no effect on Inhofe, who clanged the alarm bells still louder.

Hagel, he said, had voted against a bill labeling the Iranian Republican Guard Corps as a terrorist organization (because, by definition, it wasn’t). He’d voted against unilateral sanctions against Iran (because unilateral sanctions have no effect). He’d appeared on Al Jazeera TV and agreed with the show’s hosts that Israel had committed war crimes (the first part is true, the second part is not).

OBVIOUS IGNORANCE

On the few occasions during the session when Republican senators explored substantive issues, it was soon clear they had no idea what they were talking about.

Sen. Kelly Ayotte, R-N.H., who has often stood alongside McCain and Sen. Lindsey Graham, R-S.C., to bash President Obama on Benghazi, tried to make much of Hagel’s co-authorship of a 2012 report by an ad hoc group called the U.S. Global Zero Nuclear Policy Commission.

Ayotte expressed shock that, in the wake of North Korea’s third nuclear test, Hagel had not removed his name from this report, which called for eliminating one leg of our nuclear triad. “We have three legs to our nuclear triad,” she said (yes, senator, that’s why it’s called a “triad”), as if it were some nuclear holy trinity.

Ayotte too is new; she seems not to know what a nuclear triad is. She certainly isn’t aware that, even among conservative thinkers in the nuclear-weapons realm, the idea of scrapping one leg of the triad — namely, land-based intercontinental ballistic missiles — is at least a respectable notion.

The argument is that ICBMs are vulnerable to nuclear attack and, at the same time, tipped with multiple, highly accurate warheads that make an opponent’s ICBMs vulnerable to attack.

In other words, by their very existence, ICBMs create an incentive for both sides to launch a pre-emptive attack in the event of a crisis.

Not to sound like a Golden Age nostalgic, but there once was a time when the members of the Senate Armed Services Committee prided themselves on having an understanding of military matters.

They disagreed in their conclusions and sometimes their premises. But most of them worked to educate themselves, at least to the point where they could debate the issues, or ask questions of a general without coming off like complete idiots.

The sad thing about this new crop of senators — especially on the Republican side — is they don’t even try to learn anything; they don’t care if they look like complete idiots, in part because their core constituents don’t care if they do either.

After Tuesday’s vote, Sen. Levin adjourned the session, saying, “We thank you all, and we look forward to another wonderful year together.” The other senators laughed, but it really wasn’t funny.

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Fred Kaplan is Slate’s “War Stories” columnist and author of the book “The Insurgents: David Petraeus and the Plot to Change the American Way of War.”